Tropolone

Tropolone are the hydroxy derivatives of tropone. Particularly well known is α - tropolone, also known nor β - and γ - tropolone. The Tropolone consist of a seven-membered carbocyclic ring system having three conjugated double bonds, a carbonyl and a hydroxyl group. The Tropolone are 6π - aromatic compounds. In the α - tropolone are carbonyl and hydroxyl group in 1,2- position on the ring. The β - tropolone (1,3- substitution ) and the γ - tropolone (1,4- substitution) is of no particular importance.

Occurrence in nature

The α - tropolone ring is found in a number of natural products, eg in Hinokitiol and stipitatic acid, which was isolated from the mold Penicillium stipitatum and in the heartwood of red cedar (Thuja plicata). Purpurogallin also contains the structural motif of α - tropolone. Colchicine is the main alkaloid of Colchicum autumnale ( autumn crocus ) and can be considered as a complex α - tropolone derivative.

Synthesis

The oxidation of Cycloheptanone with selenium dioxide leads to 1,2- Cycloheptandion. The subsequent bromination - dehydrobromination in basic medium and hydrogenation leads to α - tropolone.

Properties

The α - tropolone exists in two tautomeric forms, as evidenced by Infrarotsprektren. Since the tautomeric conversion speed is very large, a structural formula is also postulated a hydrogen bond. The change of the proton from one oxygen atom to another also leads to a shift of the π - electrons of the double bonds, resulting in a newly -built seven-membered ring with 6π -electron to form a resonance-stabilized system that meets the Hückel rule, that is an aromatic:

Reactivity

α - tropolone can also - like many other aromatics - nitration and bromination. It couples with diazonium salts. By heating it is isomerized to benzoic acid.